The progress of medical bacteriology after 1880 was extraordinarily rapid, its most conspicuous accomplishment being the identification of the microbial agents of numerous severe and socially disruptive communicable diseases. The new knowledge was potentially useful knowledge and application of bacteriological understanding and techniques began almost immediately. An important field of application was the study of the movement of disease through an afflicted human population. This was the traditional territory of epidemiology. Often combined in a single person, the epidemiologist-bacteriologist attempted to exploit diverse aspects of the germ theory of disease, having recognized clearly the unique promise of a rigorous scientific foundation for long-standing public-health concerns. The focus of this proposal is historical study of the epidemiologist's appropriation and effective utilization of the new bacteriology. Supported by talented associates, Robert Koch took the lead in this endeavor. Analysis of his study of Asiatic cholera (Egypt, 1983, affording a conceptual and technical baseline; then Hamburg, 1892-1893) and typhoid fever (Ruhrgebiet, ca. 1890-1910), as well as assessment of the response of critics, will show how laboratory science was introduced into field investigation and exhibit the nature of the hygienic measures proposed and imposed as a result of these inquiries. Central issues include the notion of the healthy carrier, the restructuring of public institutions and administrative powers to accommodate new technical procedures and the creation of popular support for the enterprise. This study constitutes part of the applicant's goal of preparing a hitory of modern epidmiology, emphasis falling on the innovative years between 1850 and 1910.